This invention relates generally to apparatus, such as, a video tape recorder (VTR) of the so-called "helical-scan type", in which video or other information signals are recorded in successive parallel tracks which are skewed or extend obliquely on a magnetic tape, and more particularly is directed to an improved tracking control system by which a magnetic head or other transducer in such apparatus is made to accurately scan the track or tracks in which the video or other information signals are recorded.
In a helical-scan VTR, the magnetic tape extends helically about at least a portion of the periphery of a guide drum and is adapted to be moved or advanced in the longitudinal direction of the tape while at least a portion of the guide drum is rotated, and the transducer or magnetic head is mounted on a rotated portion of the guide drum so as to rotate with the latter and thereby repeatedly scan across the tape in a path at an angle to the longitudinal direction of the tape. During recording operation of the VTR, the angle between the scanning path, and hence each record track, and the longitudinal direction of the tape is dependent on the rotational speed of the rotary head and also the speed at which the magnetic tape is longitudinally advanced. Accordingly, if the speed and direction of advancement of the magnetic tape are not the same during the reproducing operation as during the recording operation, then the scanning path of the magnetic head during reproducing will not precisely follow or coincide with a record track on the tape during each movement of the head across the tape and, accordingly, the recorded video or other information signals may not be correctly or accurately reproduced.
Various tracking control or servo systems have been proposed for maintaining correct tracking or scanning of the record tracks by the rotary head. In the most desirable of these known arrangements, means are provided for deflecting the head in a direction normal to the plane of its rotation, that is, in a direction which is transverse in repect to the direction along each of the record tracks, and the amount of such deflection is electrically controlled during the movement of the head along each track so as to achieve accurate scanning of the latter. In published Japanese Patent Application No. 9919/1974 (Applicant: Matsushita Electric Industrial Company, Limited), it is generally disclosed to control the amount of deflection of the head in the direction normal to its plane of rotation in dependence on the difference between the speeds of advancement of the tape during the recording and reproducing operations so as to theoretically make possible the correct reproduction of the recorded video signals in the still-motion mode in which the tape is at rest, in the slow-motion mode in which the speed of advancement of the tape is, for example, 1/4 or 1/2 the tape speed for the recording operation, and in the quick-motion mode in which the speed of advancement of the tape is substantially greater than, for example, 2 times, the tape speed, for the recording operation. Further, in published Japanese Patent Application No. 117,106/1977 (Applicant: Ampex Corporation), it is disclosed to detect the amount of deflection of the head required for accurately scanning a record track thereby, and, when the detected deflection approaches the physical limit of that possible with the bi-morph leaf or other transducer deflecting device supporting the head, the electrical drive signal for the bi-morph leaf is controlled to cause the head to scan the next adjacent record track, that is, to effect a so-called "track jump". By reason of such "track jump" it is theoretically possible to effect correct slow-motion reproduction and also reverse-motion reproduction in which, for example, the tape is advanced longitudinally at the same speed as for the recording operation, but in the reverse or opposite direction.
However, in many existing tracking control or servo systems, the amplitude of the deflection of the rotary head or transducer by its transducer deflecting device is not optimized, that is, the maximum required deflection of the head in the non-normal reproducing modes, for example, the still-motion, slow-motion, fast-forward and reverse-motion modes, is not minimized. The foregoing tends to limit the permissible tape speed for reproducing in the fast-forward and reverse-motion modes, and to cause phase deviations or errors in the reproduced signals. Further, the failure to optimize the amplitude of the deflection of the rotary head or transducer by the bi-morph leaf or other transducer deflecting device disadvantageously decreases the durability of the transducer deflecting device and the speed and linearity of its response to the electrical drive or control signal therefor. Moreover, a circuit of undesirably large capacity is required for providing the electrical drive signal to the transducer deflecting device when the deflection amplitude is not optimized.
In U.S. Pat. No. 4,287,538, issued Sept. 1, 1981, and having a common assignee herewith, there is disclosed an automatic tracking control system which optimizes the amplitude of the deflection of the rotary head by its deflecting device for each of the non-normal reproducing modes, and in which a first signal is produced to correspond to the deflection of the bi-morph leaf or transducer deflecting device needed to cause the transducer or head to follow the desired track at a predetermined position therealong, a second signal is produced to correspond to the below value: ##EQU1## in which n is the ratio of the tape speed during reproducing to tape speed during recording, d is the percentage of the distance along each track from one end thereof to said predetermined position therealong and m is an integer that is no greater than n and no less than n-1, and a head position and track selection control signal is produced on the basis of the relation of the foregoing first and second signals and is applied to the transducer deflecting device so as to determine the starting position of the transducer or head and, thereby, the next desired track to be followed or scanned thereby. More particularly, in the foregoing automatic tracking control system, the drive signal applied to the transducer deflecting device comprises, in addition to the mentioned head position and track selection control signal, a dithering signal or oscillation and a track following error signal. Such error signal is derived by synchronously detecting the envelope of the reproduced output of the head with a head movement signal obtained from a head movement detector, such as, for example, a strain gauge affixed to the bi-morph leaf carrying the head, so that the head movement signal contains components corresponding to all sources of head deflection transverse to the direction along the tracks, including the head position and track selection control signal, which has a saw-tooth configuration in the non-normal reproducing modes, in addition to the dithering and error signals and mechanical vibrations of the bi-morph leaf itself. Among such components of the head movement signal, at least the components due to the head position and track selection control signal of saw-tooth configuration are not superimposed on the envelope of the signals reproduced by the head. Therefore, such components of the head movement signal due to the head position and track selection control signal are of no significance in synthesizing the error signal by synchronous detection of the envelope of the reproduced signals, but rather deteriorate the error signal thus produced. In an attempt to minimize such deterioration of the error signal, a filter has been provided in the line supplying the head movement signal to the synchronous detector for eliminating the undesired components, including those of saw-tooth configuration, from the signal passing therethrough to the synchronous detector. However, such function of the filter requires that it be carefully designed and constructed, leading to undesirable increases in the cost of the apparatus.